Relating to the mouth (4)
'relating to the mouth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for oral that I've seen before include "Exam using speech" , "Spoken, of the mouth" , "Spoken exam" , "'Spoken, not written (4)'" , "from the trap" .)
(Another definition for oscular that I've seen is " Kissing".)